L. Bell—Absolute Wave-length of Light. are 
is pressed against them with perfect uniformity by means of 
weights. An adjustable platform below carries the standards 
and objects to be measured. The ways of both carriage and 
platform had been ground till they were perfectly uniform and 
true and the working of the instrument left little to be desired 
in the way of accuracy. Throughout a long series of measure- 
ments the stops would not be displaced by so much as O-Lp if 
proper care were used in moving the carriage. The microscope 
was attached so firmly as to avoid all shaking, and was armed 
* with a half-inch objective and an excellent eyepiece micrometer. 
The objective was made specially for micrometric work and 
was fitted with a Tolles’ opaque illuminator. Measurements 
were made as follows: The standard bar, and the grating 
mounted on a polished block of speculum metal, were placed 
side by side,—or sometimes end to end,—on the platform and 
very accurately leveled. The stops were set very nearly three 
centimeters apart, one end of the grating brought under the 
microscope resting against one of the stops, and the micrometer 
set on the terminal line. Then the carriage was brought against 
the other stop and the micrometer again set. ‘The same process 
was then gone through with on three centimeters of the stand- 
ard, and then going back to the grating it was compared in the 
same manner with succeeding triple centimeters till the fifteen 
centimeter line was reached, thus eliminating the errors of the 
single centimeters and making the determination rest only on 
the fifteen centimeter line. The temperature was given by a 
thermometer placed against the standard bar or the block that 
carried the grating. In this manner each grating was repeat- 
edly compared with the first 15 centimeters of each bar, at or 
near 20°, the temperature at which the gratings had been used. 
The micrometer constant was determined by measuring tenths 
of millimeters ruled on Prof. Rowland’s engine, but in practice 
the stops were so adjusted that it was almost eliminated. Hach 
division of the micrometer head equalled 0-284 and the proba- 
ble error of setting was less than half that amount. 
All measurements were reduced to 20° ©. as in case of the 
angular determinations. The line along which the linear meas- 
ures were made was that formed by the terminations of the 
rulings. It therefore was necessary to know very exactly the 
angle between this line and the direction of the individual rul- 
ings, in other words the angle between the line of motion of the 
grating and the direction of the diamond stroke in the dividing 
engine. This was ascertained by means of two test plates each 
some twelve cm. long ruled in ems. and then superimposed line 
for line. By measuring the minute distances between each end 
ofa pair of superimposed lines, the length of the lines and the 
amount by which their ends overlapped at each end of the test 
